The semiconductor manufacturing industry requires piping or systems of high integrity for the transport of high purity gases for processing of semiconductors. These systems must be free of gas leaks to prevent entry by backflow of atmospheric contaminants such as moisture, oxygen and particulates. The industry is currently requiring that installations be evaluated for leakage, and that the leakage be less than a specific rate, defined in terms of a volume rate of flow of a specified fluid under specified conditions. A common specification is that the system be demonstrated to have a leakage rate of less than 2.times.10.sup.-8 atm cc/sec (2.03.times.10.sup.-3 Pa cc/sec) when charged with helium gas to the normal operating pressure of the system.
While many methods of detecting leaks from a vessel are known, methods of quantitatively measuring leakage from a vessel are few. In the prior art, the leakage rate of a vessel which can be pressurized has been commonly measured by a technique known as accumulation testing. The method is described in the Nondestructive Testing Handbook, Second Edition, Volume One, Leak Testing, pages 481-482. The vessel to be evaluated is placed in a sealed enclosure termed an accumulation chamber. A standard helium leak apparatus is connected in a manner that will allow helium gas to pass at a known rate into the accumulation chamber for calibration purposes. A helium leak detector probe is inserted into the accumulation chamber. The leak detector reading is recorded as a function of time while the standard leak helium inflow rate is in effect, to obtain a calibration curve.
After the calibration data have been acquired, the standard leak is removed from the accumulation chamber, and the chamber is purged. The test vessel is then pressurized with helium, and the leak detector reading is recorded as a function of time. The vessel leakage curve is then compared with the standard leak curve to obtain a quantitative estimate of the leakage experienced from the vessel.
This method has several undesirable features. One is that the air in the accumulation chamber contains its naturally occurring 5 ppm helium as a background or interference. This background reduces the sensitivity and accuracy of the measurement of leakage of helium from the test vessel. Another is that the diffusion of tracer gas through the air being slow requires mechanical mixing of the air within the chamber for uniform dispersion of the tracer and accurate results. Another is that a vessel such as a valve in a pipeline is difficult to accommodate within a chamber. The pipeline can be allowed to protrude from the chamber, but a seal is necessary at the protrusions to avoid entry of atmospheric air into the chamber or escape of gas from the chamber. Still another undesirable feature is the lengthy time required for the performance of the leakage rate measurement because of the need for a calibration with a standard leak apparatus for comparison with the vessel leakage.